Exhaust components, such as exhaust manifolds, exhaust pipes, converter cases, and mufflers used under the exhaust environment of automobiles, are required to be excellent in terms of thermal fatigue resistance or oxidation resistance (hereinafter, both properties are collectively referred to as “heat resistance”). For applications requiring such heat resistance, Cr containing steels, to which Nb and Si are added, such as Type 429 (14Cr-0.9Si-0.4Nb), are currently used frequently. However, when an exhaust gas temperature increases so as to exceed 900° C. with an improvement in engine performance, the thermal fatigue resistance of the Type 429 becomes insufficient.
To deal with this problem, Cr containing steels whose high temperature proof stress has been improved by adding Nb and Mo, SUS444 (19Cr-0.5Nb-2Mo) specified in JIS G4305, ferritic stainless steels to which Nb, Mo, and W have been added, etc., have been developed (e.g., Japanese Unexamined Patent Application Publication No. 2004-018921). However, due to the unusually steep increase in the price of rare metal raw materials, such as Mo or W, these days, the development of materials having heat resistance equivalent thereto using inexpensive raw materials has been increasingly required.
As materials that are excellent in terms of heat resistance and that do not contain expensive elements, such as Mo and W, for example, WO 2003/004714 pamphlet discloses a ferritic stainless steel for automobile exhaust air passage components in which Nb: 0.50 mass % or lower, Cu: 0.8 to 2.0 mass %, and V: 0.03 to 0.20 mass % are added to 10 to 20 mass % Cr steel, Japanese Unexamined Patent Application Publication No. 2006-117985 discloses a ferritic stainless steel excellent in terms of thermal fatigue resistance in which Ti: 0.05 to 0.30 mass %, Nb: 0.10 to 0.60 mass %, Cu: 0.8 to 2.0 mass %, and B: 0.0005 to 0.02 mass % are added to 10 to 20 mass % Cr steel, and Japanese Unexamined Patent Application Publication No. 2000-297355 discloses ferritic stainless steels for automobile exhaust air parts in which Cu: 1 to 3 mass % are added to 15 to 25 mass % Cr steel. The respective steels have improved thermal fatigue resistance by the addition of Cu.
We found that, when Cu is added as in the techniques of the known publications mentioned above, the fatigue resistance improves, but the oxidation resistance of the steel itself decreases and, thus, the heat resistance generally deteriorates.
It could therefore be helpful to provide a ferritic stainless steel excellent in terms of both oxidation resistance and thermal fatigue resistance without adding expensive elements such as Mo or W by developing a technique for preventing reduction in oxidation resistance by the addition of Cu.